The present invention relates generally to a semiconductor laser device having an active region of a quantum well structure of an InGaP crystal, and a DVD system using the semiconductor laser.
Semiconductor laser devices have excellent characteristics, such as small size, steadiness, easiness of modulation, operability at a low voltage and a low current, and a long life in comparison with gas lasers and solid lasers. Therefore, semiconductor laser devices are not only substituted for gas lasers in fields which have been practically achieved, but there are often also application fields in which can be first practically achieved by semiconductor laser devices. As examples thereof, there are optical communication systems and optical information processing systems.
However, conventional semiconductor laser devices can not sufficiently clear specifications necessary for these application fields.
For example, a DVD system, such as a DVD-ROM (Digital Versatile Disc-Read Only Memory) and DVD-RAM(Random Access Memory), has been noticed as an optical information processing system capable of performing rapid access at a large capacity. It has been expected that a semiconductor laser device is used for a light source for an optical pickup head of the DVD systems.
However, according to the standard for light sources of the DVD systems, which has been standardized all over the world, it is required that the light sources have an oscillating wavelength of 650 nm, a very low threshold current, a high efficiency, stable temperature characteristics, a high maximum oscillating temperature (Tmax) and so forth.
On the other hand, a semiconductor laser device using an InGaAlP crystal, which lattice-matches with a GaAs substrate, practically has an oscillating waveband which is in the range of from 630 nm to 700 nm. In recent years, semiconductor laser devices using a quantum well structure, which uses an InGaP semiconductor layer having a thickness of a few nm as a well layer so that an active region has quantum effects, have been produced.
However, conventional semiconductor lasers having an oscillating wavelength of 650 nm have a high threshold current of about 60 mA and a low maximum oscillating temperature (Tmax) of about 80.degree. C. For that reason, conventional semiconductor lasers do not meet the specifications of the DVD standard, so that it is required to much improve the characteristics of the semiconductor lasers.